1. Field of the Invention
This invention relates to the utilization and disposal of shot coke. More particularly, the invention relates to a process of gasifying shot coke and thereby consuming this undesirable component of coke fuels while simultaneously generating steam and producing a pressurized synthetic gas stream.
Shot coke is an undesirable form of coke that is generated intermittently in delayed cokers. Delayed cokers are designed for the manufacture of sponge coke. More specifically, shot coke is an agglomeration of spherical coke particles of uniform size, approximately 4 mm in diameter, in which the spherical coke particles are fused together at their points of contact and embedded in a matrix of finer coke particles to form coke chunks of varying size. These coke chunks have many voids and are permeable. Upon grinding, the coke chunks are easily broken up to yield two types of coke particles, i.e., a powdery coke material and the 4 mm spherical particles of shot coke. Whereas sponge coke manufactured in delayed coking is amenable to grinding, the spherical shot coke particles are not. A standard method for determining ease of grindability of coal is the Hardgrove-Machine method, ASTM D409-71. In this particular test, the larger the resultant numerical value, the easier it is to grind the coke. Sponge coke has a Hardgrove grindability index of about 70. Shot coke has a Hardgrove grindability index of about 20. This inability to be broken into finer particles renders shot coke an undesirable component in sponge coke and if such material can be sold at all, shot coke brings a lower price than the sponge coke.
While the shot coke particles are difficult to grind further, these uniformly sized particles can be readily fluidized. Accordingly, it would be worthwhile to take advantage of the physical properties of the shot coke and, at the same time, eliminate any economic disadvantage associated with the production of this material.
2. Discussion of the Prior Art
It is, of course, well known to gasify coal by introducing oxygen and steam into a fluidized bed of coal particles and thereby produce a synthetic gas comprised of carbon monoxide and hydrogen. For example, in U.S. Pat. No. 4,198,212, issued Apr. 15, 1980, to Tsao, coal is gasified to produce a coal gasification effluent and a char residue. The char residue is cooled by indirect heat transfer in a separate char bed, and the coal gasification effluent is passed through the cool char bed to effect cooling of the coal gasification effluent, with tars and oil present in the effluent being absorbed by the char bed. Further cooling of the char bed is provided by a heat recovery system including the generation of steam by indirect heat exchange with boiler feed water.
Integrated coking and gasification processes are disclosed in U.S. Pat. Nos. 4,055,484, 4,077,869, and 4,219,402 in which separate coking and heating zones and a single gasifying zone are provided. These patents are not concerned with taking advantage of the physical properties of shot coke and finding a useful means of disposing of this otherwise undesirable component in coke fuels.
U.S. Pat. No. 2,903,980, issued Sept. 15, 1959, to Gorin discloses the combustion of finely divided low volatile carbonaceous solid fuels in a two-step combustion process comprising partial incomplete combustion of carbonaceous solids to form char and combustible gases and consuming the combustion products in a polarized fuel burner. Fresh carbonaceous solids may also be introduced into the fuel burner. The Gorin patent is not concerned with the problem of eliminating the economic disadvantage associated with shot coke production and does not disclose an overall combustion process which not only takes advantage of the physical properties of shot coke to dispose of this material, but also takes advantage of the various chemical reactions which take place in accordance with the present invention to produce a pressurized synthetic gas useful to drive power turbines and generate steam and to generate methanol and other hydrocarbons.